The present invention relates to microlithographic systems and more particularly to a lens technique which reduces errors caused by barometric changes in the atmosphere in which the microlithographic system operates.
While it has previously been recognized that changes in barometric pressure effect the focus and magnifying properties of lenses, correction for such effects have seldom been attempted or, if attempted, has resulted in extremely expensive and complicated pressure control systems. For example, in the Brouwer U.S. Pat. No. 3,498,695, and in the McCrobie et al. U.S. Pat. No. 4,331,388, systems are disclosed in which the interior of a lens is pressurized with a heavy or dense gas in order to control or utilize the changes in refractive index which accompany such pressurization. A microlithographic system manufactured by Nikon Precision, Inc., is believed to incorporate a automated barometric compensator of the type disclosed in the McCrobie et al. U.S. Pat. No. 4,331,388, with interior lens pressures being adjusted under computer control. In such prior art systems, it is believed that the basic lens design must be substantially modified in order for the control systems to operate as intended.
In sharp contrast with these prior art approaches, the present invention contemplates filling the interior spaces of a microlithographic projection lens with a very low density gas, supplied essentially at atmospheric pressure. As is understood by those skilled in the art, such gases have an index of refraction which is very close to unity so that the barometric effect on the refractive properties of any gas/glass interface is minimal.
Among the several objects of the present invention may be noted the provision of a lens technique for microlithography which reduces the effects of barometric changes in the atmosphere in which the microlithographic systems operates; the provision of such a technique which does not require elaborate pressure control systems; a provision of such a technique which provides a high degree of stability in magnification and focus in a microlithographic lens system; the provision of such a system which does not substantially constrain lens design; the provision of such a technique which is easily implemented, which is highly reliable and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.